This paper analyses the politics of regulatory expansion within the diagnostics sector. Since 1990, an informal, clinician-led process of diagnostic innovation within the UK NHS has been challenged by new mechanisms for the evaluation of diagnostics. We describe these diagnostic reforms as a process of fragmented regulatory expansion. New governance mechanisms function as regulatory niches: discrete spaces within an overarching sociotechnical regime. The boundaries of regulatory niches are organisational and epistemological. Organisational boundaries map onto established communities of practice that constitute the regulatory target; epistemological boundaries are defined by distinctive evaluation frameworks. Niches are also distinguished by their outcomes (rate of positive decisions) and their origins. Niche formation was triggered by five drivers: public scandal; technological change; marketisation; institutional isomorphism; and transnational policy transfer. Each niche was triggered by a unique confluence of these drivers, but common to all were historic shifts in healthcare politics, as the rise of evidence-based medicine intersected with the centralising impulse of the regulatory state, which encroached on clinical autonomy in a contest for power that is increasingly mediated by influential non-governmental organisations.